1. Field of the Invention
This invention relates to ferromagnetic powders for a magnetic recording medium and method for preparation thereof, and in particular, to novel ferromagnetic powders and a method for preparation thereof.
2. Description of the Prior Art
Ferromagnetic powders which have heretofore been used in magnetic recording media include .gamma.--Fe.sub.2 O.sub.3, Co-containing .gamma.--Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing Fe.sub.3 O.sub.4 and CrO.sub.2. However, these ferromagnetic powders are not very suitable for magnetic recording of signals of a short recording wavelength (about 10 .mu. or less). This is because the magnetic characteristics such as the coercive force (Hc) and the maximum residual magnetic flux density (Br) of these ferromagnetic powders are not sufficient for use in high density recording. Recently, extensive development of ferromagnetic powders having magnetic characteristics suitable for high density recording has been carried out. One substance investigated is a ferromagnetic powder. The powder is made of a metal or a metal alloy. Metals which are mainly used include iron, cobalt and nickel, and other elements such as chromium, manganese, rare earth elements and zinc are optionally added thereto.
The following methods are known for preparation of ferromagnetic powders.
1. A salt of a ferromagnetic metal with an organic acid is pyrolyzed and reduced with a reductive gas, for example, as disclosed in Japanese Patent Publication Nos. 11412/61, 22230/61, 8027/65, 14818/66, 22394/68 and 38417/72, and Japanese Report of Association of Tohoku University Wire Conversation, Vol. 33, No. 2, page 57 (1964).
2. A needle-shaped oxyhydroxide, or a derivative thereof containing other metals, or a needle-shaped iron oxide obtained from these oxyhydroxide substances is reduced, for example, as disclosed in Japanese Patent Publication Nos. 3865/65, 20939/64 and 29477/72, German Patent Application (Laid-Open to Public Inspection) No. OLS 2,130,921, British Patent No. 1,192,167 and U.S. Pat. No. 3,681,018.
3. A ferromagnetic metal is evaporated in an inert gas, for example, as disclosed in Japanese Patent Publication No. 27718/72 and Ohyo Butsuri entitled Applied Physics, Vol. 40, No. 1, page 110 (1971).
4. A metal carbonyl compound is decomposed, for example, as disclosed in U.S. Pat. Nos. 2,983,997; 3,172,776, 3,200,007 and 3,228,882.
5. A ferromagnetic metal is deposited by electrodeposition using a mercury cathode, and then separated from the mercury, for example, as disclosed in Japanese Patent Publication Nos. 15525/64 and 8123/65, and U.S. Pat. No. 3,156,650.
6. A salt of a ferromagnetic metal is reduced in a solution thereof, for example, as disclosed in Japanese Patent Publication Nos. 20520/63, 26555/63, 20116/68 and 41718/72, U.S. Pat. Nos. 3,567,525; 3,663,318; 3,661,556; 3,669,643; 3,672,867 and 3,756,866, and German Patent Application (Laid-Open to Public Inspection) OLS No. 2,132,430.
This invention is concerned with Method (6) wherein a salt of a ferromagnetic metal is reduced in a solution thereof, and in particular, with a method wherein a borohydride compound or a derivative thereof is used as a reducing agent.
The Method (6) wherein a salt of a ferromagnetic metal is reduced in a solution thereof with a borohydride compound or a derivative thereof has some defects as mentioned below.
In general, the reaction is carried out in a magnetic field to impart a magnetic anisotropy to the reaction system whereby granular particles are linked and shape anisotropy is imparted thereto. However, the linked particles are cleaved during admixture with a binder and dispersion therein, often resulting in a decrease of the shape anisotropy of the particles, a deterioration of the magnetic orientation and a degradation of the squareness ratio (Br/Bs) of the formed magnetic recording tape.
In addition, the powders obtained according to this method are less resistant to moisture, particularly when Fe is included, and are gradually oxidized even at normal temperature, in an extreme case, resulting in a loss of the magnetic character thereof. According to this method, furthermore, since the surface activity of the particles obtained is extremely high, the particles are highly active, which is industrially disadvantageous from the standpoint of their production.
Prior to the present invention various studies have been made to improve the above described defects. For example, U.S. Pat. Nos. 3,535,104, 3,567,525, 3,661,556 and 3,663,318, and Japanese Patent Publication No. 20520/63 disclose a Fe--Co--Ni--B system or a Fe--Co--Ni--P system containing Mn or Cr. Japanese Patent Publication No. 7820/72 discloses the addition of Mn, Cu or Ag to a Fe--Co--Ni--B system. U.S. Pat. No. 3,206,338 discloses a Fe--Co--B system containing Cr. U.S. Pat. Nos. 3,669,643, 3,672,867 and 3,756,866 disclose a Co--P system or a Co--B system containing Cr, Zn, Pt, As, Ca, Ge or Japanese Patent Publication No. 20116/68 suggests a method for saponification of the surface of metal powders with higher fatty acids.
Further, the addition of Cr is already known, as disclosed in Japanese Patent Publication No. 20520/68. In the conventional method, however, wherein Cr is previously added to the reaction bath to prepare ferromagnetic powders, the Cr added is substantially uniformly distributed in the formed powders and the product is prepared in the form of an alloy, solid solution or the like. The coercive force of the formed ferromagnetic powders decreases as the content of the Cr therein increases, and thus it was impossible to make the best use of the essential characteristics of the ferromagnetic powders.